Here's a function that implements this logic. You pass it the number of players and the round number and it returns a list of numbers. Each pair of numbers is a matchup for that round. (A pair of say 1,4 means "Player 1 plays Player 4".)

# Function for determining the matchups in a round-robin
# tournament round, given the number of players and the
# round number.
sub matchups {
    my $players = shift;
    my $round = shift;
    
    # Sanity check the round number
    if ($round < 1 || $round > $players-1) {
        return;
    }

    my @list = (1, 1+$round .. $players, 2 .. $round);
    my @pairs;
    for (0 .. $#list / 2) {
        push @pairs, $list[$_], $list[$players-1 - $_];
    }

    return @pairs;
}
[download]

If there are an odd number of players, the last pair will be the same number twice (meaning "Player 4 plays Player 4"), which is essentially a bye.

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